Apparatus and techniques have been disclosed in the prior art by which coatings can be produced upon a variety of objects, by exposing them to a cloud of solid particles. Such a cloud may be generated by charging air upwardly through a bed thereof, as in conventional fluidized bed equipment, or by the use of spray systems and the like. Depending upon the nature of the particulate material employed, adhesion to the object may be promoted by preheating, adhesive precoating, or by other means, and a variety of techniques can be used to convert the particles unto a unified, continuous coating, again depending upon the characteristics of the coating material employed.
Although many different types of objects (such as continuous lengths of wire, tubing, and the like, as well as individual objects which may be passed into or adjacent the cloud of particles) may be coated in such a manner, difficulties are encountered in attempting to produce uniform coatings upon objects having vertical dimensions in excess of a certain value. This is due largely to the rarefaction that occurs in the upper regions of the cloud, which produces a vertical gradient of particle concentration, thereby causing objects which have surfaces at substantially different heights to acquire heavy deposits upon their lower portions, while developing progressively thinner coatings on the upper surfaces. These effects may be most pronounced when the particle cloud is produced in fluidized bed equipment, and especially when fusion is relied upon for adherence of the particles; in the latter event, the pre-existence of coating material on the surface of the object does not effectively limit the thickness of the deposit ultimately produced, as is the case when, for example, adhesion is electrostatically induced.
Accordingly, it is the principal object of the present invention to provide a novel apparatus and system in which articles having a significant vertical dimension may be uniformly coated with particles from a cloud thereof.
It is also an object of the invention to provide such an apparatus and system which are highly efficient, safe, and convenient to operate, and which are at the same time relatively simple and inexpensive to produce.
Another object is to provide a novel method by which an object of significant vertical dimensions may be uniformly coated by exposure to a cloud of particles.